This invention relates to liquid crystal displays wherein the pixels are controlled by varistors.
Various types of liquid crystal displays are known. The twisted nematic type relies on the ability of liquid crystals to rotate the polarization plane of incident light, thus causing the light to be absorbed or not by polarizers associated with the display. The encapsulated liquid crystal type relies on the ability of encapsulated liquid crystal material to switch from a state in which incident light is highly absorbed and/or scattered to one in which the light is substantially transmitted.
Where the display is used to depict simple alphanumeric characters, for example via the familiar seven-segment, figure-eight pattern found in calculators and watches, it is feasible to directly address each pixel in the display--that is, to provide each pixel with its own set of electrode leads. But where the display must depict complex images such as graphics or video images, a large number of pixels is required, and it becomes impractical to directly address each one. A display having pixels arranged in M rows and N columns has M.times.N pixels, thus requiring M.times.N sets of individual leads for direct addressing. As the pixel density and/or the size of the display increases, this number rapidly becomes unmanageable.
Multiplexing provides a method of addressing each pixel, but with a much lesser number of electrode leads. In its most elementary form, multiplexing uses a set of M row electrodes in conjunction with a set of N column electrodes. By applying the proper electrical signals to, for example, the 5th row and 8th column electrodes, the pixel at the 5th row and 8th column can be switched on and off. In this way, the number of electrode leads can be reduced from M.times.N to M+N. However, in this simplest form of multiplexing, the adjacent pixels are not independent of each other. When a voltage sufficient to switch the 5th row-8th column pixel is applied, the adjacent pixels (e.g., the 4th row-8th column pixel) also experiences a substantial voltage and can also be inadvertently switched, at least in part, leading to cross-talk between adjacent pixels.
It has been proposed to use a nonlinear electrical component, such as a metal-insulator-metal (MIM) diode or a thin film transistor (TFT) associated with each pixel to control the switching of each pixel and to eliminate cross-talk. Another nonlinear element is a varistor, which has a voltage-current relationship described by the equation ##EQU1## where I is the current flowing through the varistor; V is the voltage across the varistor; C is a constant which is a function of the dimensions, composition, and method of fabrication of the varistor; and .alpha. is a constant which is a measure of the nonlinearity of the varistor. A large .alpha., signifying a large degree of nonlinearity, is desirable. High quality varistors typically have an .alpha. between 20 and 50.
Varistors have also been proposed as the switching elements in liquid crystal displays. Castleberry, U.S. Pat. Nos. 4,233,603 (1980), and Hareng et al., 4,535,327 (1985) disclose the use of varistors in multiplexed liquid crystal displays. Yoshimoto et al., EP 337,711 (1989), disclose varistors as switching elements in multiplexed encapsulated liquid crystal displays.
The prior art varistor driven liquid crystal displays have several limitations. Liquid crystal displays operate at relatively low voltages, ranging from a few volts for the twisted nematic ones and from about 20 to about 100 volts for the encapsulated liquid crystal ones. For liquid crystal displays operating at video rates--for example in television sets--it is necessary to drive the pixels "off" to achieve rapid switching, instead of waiting for the natural decay to the "off" state according to the RC constants of the liquid crystal material. This can be effected only by matching V.sub.on (the voltage at which the pixel switches) with V.sub.t (the threshold voltage of a varistor, below which the varistor is highly resistive and above which the varistor's resistance drops dramatically). If one attempts to match V.sub.on and V.sub.t by increasing the thickness of a liquid crystal cell and thereby its operating voltage, a twisted nematic cell's response time increases, up to 3-4 fold, making this approach unsatisfactory for video rate applications. Conversely, if one attempts to reduce V.sub.t by reducing the thickness of the varistor, one runs into the problem of reproducibly making very thin varistor elements which tend to be fragile.
Levinson, in U.S. Pat. No. 4,364,021 (1982), discloses a varistor having a recessed region on one of its planar surfaces, to provide a region of reduced thickness and consequently lower breakdown voltage, while retaining the structural strength of a thicker varistor. However, Levinson relates to the preparation of individual varistors. For a multiplexed liquid crystal display, where a large array of varistors is required, one would then have to mount these individual varistors onto a supporting base, an inefficient step.
Kujawa et al., U.S. Pat. No. 3,195,091 (1965), discloses a silicon carbide non-linear resistor having plural leads attached to recesses on one surface thereof and a single lead attached to the other surface thereof.
Another critical requirement in varistor driven liquid crystal displays is that the varistor material be stable through a very large number of cycles--over billions of cycles. The prior art varistor-driven liquid crystal displays employ conventional varistor materials, most of which have been developed for applications such as surge arrestors, in which the number of on-off cycles above the threshold voltage is small.